Onions Based on Universal Re-encryption – Anonymous Communication Immune Against Repetitive Attack

Abstract

Abstract. Encapsulating messages in onions is one of the major techniques providing anonymous communication in computer networks. To some extent, it provides security against traffic analysis by a passive adversary. However, it can be highly vulnerable to attacks by an active adversary. For instance, the adversary may perform a simple so-called repetitive attack: a malicious server sends the same massage twice, then the adversary traces places where the same message appears twice -revealing the route of the original message. A repetitive attack was examined for mix-networks. However, none of the countermeasures designed is suitable for onion-routing. In this paper we propose an "onion-like" encoding design based on universal reencryption. The onions constructed in this way can be used in a protocol that achieves the same goals as the classical onions, however, at the same time we achieve immunity against a repetitive attack. Even if an adversary disturbs communication and prevents processing a message somewhere on the onion path, it is easy to identify the malicious server performing the attack and provide an evidence of its illegal behavior.

Attacking and Repairing the Improved ModOnions Protocol

“…-λ is a static parameter of the protocol, -E denotes encryption scheme with properties as in [9,11], -for each block one of destination keys x i is used and only once; moreover, it is the destination key of the final recipient of the information stored in the block.…”

“…It is assumed that malicious node, once caught is permanently excluded from the protocol. Detailed description can be found in [9].…”

“…The fact remains, however that TOR is the most secure implemented solution. ModOnions [9] take an alternate approach, using Universal Re-Encryption (URE) [8] to re-randomize the messages ("onions"), such that duplicate copies of an onion cannot be linked. Re-encryption of such onions is possible, since instead of a single onion with a message hidden at each "layer" of the onion, there is a group of onions to be processed together, each encoding a different routing information for a path.…”

Anonymous Communication with On-line and Off-line Onion Encoding

“…Papers [5,10] introduce new encoding techniques to onions based on universal reencryption schemes [9]. The idea is that the components processed by the servers can be re-encrypted without any knowledge of the contents and the recipient.…”

“…New results In our paper we explore new possibilities for design of anonymous communication protocols based on onions encoded with universal re-encryption schemes [10]. First we propose an off-line protocol that allows to prepare a route of a message in advance -the onion routes (or their parts) are created by third parties as a kind of general service.…”